SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  cscart_discussion 
  INNER JOIN cscart_products as p ON (
    cscart_discussion.object_id = p.product_id
  ) 
  INNER JOIN cscart_discussion_posts as dp ON (
    cscart_discussion.thread_id = dp.thread_id 
    AND cscart_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    13416, 13417, 13418, 13452, 13419, 13420, 
    13453, 13502, 13421, 13501, 13422, 
    13454, 13455, 13423, 13456, 13457, 
    13458, 13459, 13505, 13424, 13496, 
    13425, 13426, 13495
  ) 
GROUP BY 
  p.product_id

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.015057387,
    "filesort": {
      "sort_key": "p.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "dp",
              "access_type": "index",
              "possible_keys": ["thread_id", "thread_id_2"],
              "key": "thread_id_2",
              "key_length": "6",
              "used_key_parts": ["thread_id", "status"],
              "loops": 1,
              "rows": 2,
              "cost": 0.00693929,
              "filtered": 100,
              "attached_condition": "dp.`status` = 'A'",
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "cscart_discussion",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "object_id"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["thread_id"],
              "ref": ["u508912950_dev_2025.dp.thread_id"],
              "loops": 2,
              "rows": 1,
              "cost": 0.00342728,
              "filtered": 100,
              "attached_condition": "cscart_discussion.object_type = 'P'"
            }
          },
          {
            "table": {
              "table_name": "p",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["u508912950_dev_2025.cscart_discussion.object_id"],
              "loops": 2,
              "rows": 1,
              "cost": 0.00342728,
              "filtered": 100,
              "attached_condition": "p.product_id in (13416,13417,13418,13452,13419,13420,13453,13502,13421,13501,13422,13454,13455,13423,13456,13457,13458,13459,13505,13424,13496,13425,13426,13495)"
            }
          }
        ]
      }
    }
  }
}